潮流作用下河口是沙运动二维数学模型

对于粉沙淤泥质河口和海岸,海底泥沙受潮流作用主要以悬沙形式输运。在这样的海区建港与疏浚航道,需要首先进行泥沙淤积问题的研究。本文采用潮流作用下不平衡方程式、挟沙能力公式和起动流速公式,建立了潮流作用下河口悬沙运动二维数学模型,在对二维悬沙不平衡输沙方程和海底变形方程进行离散时直接采用显式迎风格式,得到了较好的结果。在此基础上,将该模型应用于实际水域,结果表明,该数学模型能够模拟河口的悬沙运动规律和冲淤变化,对于水流较大的海域该模型有一定的应用价值     

海南东方西部海域沙波和沙脊分布特征

海南东方西南部海域海底发育大量沙波、沙脊,严重影响该海域海底工程安全。为探明该区域的海底地貌概况及微地貌特征,本论文通过侧扫声呐、浅地层剖面等方法进行了探测。探测结果表明：区域内沙波、沙脊十分发育;受沙波、沙脊等地貌单元的影响,该区域内水深多变化,地形条件复杂。根据沙波发育形态、规模等特征,共识别出沙脊、巨型沙波、大型沙波、中型沙波、小型沙波以及沙纹六个地貌类型。根据沙波沙脊发育特征及其空间组合关系推测,研究区内的沙脊发育于低海面时期的浅水高能环境,活动性较弱,沙波则主要形成于末次海侵以来高海面低能环境中,当前仍处于发育过程中。该研究结果可为区域内海底管线、桩基础等海洋工程设计、防护工作提供参考。     

北部湾东南海域海底沙波发育分布特征及控制因素

北部湾东南海域海底发育大量沙波,利用最新获得的多波束测深、沉积物和水动力数据对沙波形态和分布特征进行综合分析,探讨沉积物特征、水动力条件,海平面变化等不同控制因素对海底沙波发育及分布的影响。结果表明研究区沙波广泛分布于沙脊顶部、沙脊槽中线附近及沙脊槽北端,沙波呈现远岸区尺度较小、对称性较好,近岸区尺度较大、多不对称的分布特征。海底沙波的发育和分布受现代潮流作用和沉积物特征的共同控制。潮流通道及流速的分布情况与沙波形态、规模的差异性分布一致。表层沉积物具有"远岸细、近岸粗"的特点,与沙波远岸尺度小、近岸尺度大的分布规律有较好的对应关系。此外,研究区出现有近对称形态的沙波,可能为海平面变化期间多期潮流共同作用形成的残余沙波。     

海南东方近岸海底活动沙波的地球物理特征及其迁移机制

海南东方近岸海底发育有大量沙波,利用多波束测深、侧扫声呐、浅地层剖面、单道地震资料综合分析了活动沙波的地球物理特征,探讨了沙波的分布特征、迁移机制、活动性及形态演变特征。结果表明,研究区海底沙波分布和规模具有显著空间差异性,大中型沙波主要发育于沙脊上,小型沙波主要发育于沙脊两侧,坑槽区发育近对称沙波,研究区西南部沙波不发育。受潮流和科氏力约束,在海底沙脊西侧沙波迁移方向主要为向北(略偏东),在沙脊东侧主要为向南(略偏西);受地形制约,坑槽区近对称沙波迁移可能停止或方向发生改变。沙波活动性强的标志主要包括:(1)形态呈不对称的"脊尖槽缓",(2)叠置小沙波与沙纹发育,(3)浅部有透明层,(4)陡坡面反射模糊,(5)内部斜交前积结构。分析认为,沙波活动性与其形态密切相关,包括弱运动、强运动、不运动3个演变阶段。     

粉砂质海岸大风天泥沙运动研究

根据黄河三角洲五号桩海域6级大风前后现场悬沙、沉积物和实测水流资料,研究了淤泥质粉砂海岸大风过程后泥沙分布特征和运动规律。发现大风天气海底泥沙对波浪作用反应灵敏,悬沙和底沙分布规律与波浪作用吻合,为波浪作用下海底泥沙运动的理论研究以及海区工程开发提供了科学依据。     

北部湾东侧莺东沙脊及其在管线工程中的负面作用

北部湾东侧莺东潮流沙脊是整个琼西南沙脊群之一,沙脊从感恩角岸外发育最好,至东方岸外已趋缓和。根据海底特征,由海向岸分为深水平坦段、沙波沙脊段和近岸段共3个区段。沙波沙脊区段主要分布4条沙脊,这些沙脊大都呈NS或NW—SE走向,与主水流平行,呈不甚规则的长条状,长约45km,宽度较大。脊槽高差约20~30m,脊顶水深5~8m,沙脊槽向不对称,向海侧缓,向陆侧陡。成因上属于全新世低海面形成的连滨沙脊,受现代浪流修蚀改造而成。这些沙脊的基础部分基本稳定,只在暴风浪或强潮流情况下表面活动性泥沙做少量迁移。2003年铺设的东方1-1气田输气管线路由经过北部湾莺东沙脊。通过对中国海洋大学东方1-1海底管线多年调查资料分析认为,本区沙波沙脊海底管线被严重掏空,已发现的130余掏空悬跨点平均长达33m,深度最大可达1m。管线掏空可归因于底形特征动态变化和管线改变海底水动力环境的综合作用。     

海岸悬沙运移数学模型

根据适合大范围缓变地形情况下,不恒定、非均匀流场中随机波折绕射联合数值模式及波浪作用下的二维浅水环流方程和悬沙扩散方程,建立了波浪、潮流共同作用下二维悬沙数学模型,并将该模型应用于渤海湾北部海域,进行了波、流共同作用下航道疏浚弃土的悬沙扩散、运移及海床演变的数值模拟,为工程单位决策提供科学依据     

南海北部沙波区海底强流的内波特征及其对沙波运动的影响

2008年3月6日至2008年4月9日, 在南海北部外陆架与陆坡上的沙波区进行了海底流速的连续观测，观测结果表明潮流与海流较弱，但时有流速达30—77cm.s-1的海底强流发生。强流方向与南海北部内波传播方向相对应，多分布在偏NW向与偏SE向。偏SE向流强于偏NW向流，与内波在传播方向上的下坡流大于上坡流的特征一致。对流速序列进行了旋转功率谱分析，结果表明，高于M2分潮的频率中，众多的振荡分量具有内波流性质，说明阵发性强流为内波所致。采用观测流速计算了沙波的移动速度，计算结果得出强流能起动海底泥沙，由于NW向传播（上坡方向）的内波导致了SE向（下坡方向）的净流动，沙波偏SE向移动，但沙波移动速度不大，小型沙波移动速度小于1.6m.a-1。采用潮流、风暴潮耦合模型计算了强台风驱动的海底流速过程，表明潮流、风暴潮耦合也能移动海底沙波，但沙波移动方向与台风路径相关，不一定为SE向，且移动距离更小，潮流、风暴潮耦合不是沙波移动的主要动力机制。     

海底沙波迁移过程原位观测简易装置设计与试验

海底沙波的迁移对海底基础工程具有潜在危害,成为人们日益关注的热点。目前对海底沙波迁移过程的研究缺乏原位长期观测数据的支撑,设计一种海底沙波迁移过程的原位观测装置,通过水压差与高程差变化的对应关系来确定沙波的形态变化和迁移过程。并且在室内波浪水槽中对装置观测的可行性和观测结果的准确性进行验证,同时在仰口湾潮滩进行现场观测应用。该装置室内试验测量计算得到沙波表面高度变化5.9 cm,而观测得到沙波表面高度实际变化5.5 cm,两者吻合程度很好。试验结果表明,这套海底沙波迁移过程原位观测装置能对沙波外部形态变化进行测量且准确度较高,为海底沙波迁移的观测提供了新方法。     

波流联合作用下的海底沙波移动对海底底床稳定性影响的研究进展

海底沙波常见于潮流作用较强的内陆架海域,是一种脊线与潮流流向垂直的微地貌形态,一般在40～100 cm/s的流速形成。沙波既可大面积自成体系,也可分布在沙脊的坡面上,呈不对称形态。泥沙的粒度、沉积物供给以及水深[1]等因素对海底沙波的发育都有影响。     

Simulating offshore sand waves

Sand waves form a prominent regular pattern in the offshore seabeds of sandy shallow seas and pose a threat to a range of offshore activities. A two-dimensional vertical (2DV) flow and morphological simulation model describing the behaviour of these sand waves has been developed. The simulation model contains the 2DV shallow water equations, with a free water surface and a general bed load formula. The water movement is coupled to the sediment transport equation with a seabed evolution equation. The domain is non-periodic in both directions. The spatial discretisation is performed by a spectral method based on Chebyshev polynomials. A fully implicit method is chosen for the discretisation in time. Firstly, we validate the simulation model mathematically by reproducing the results obtained using a linear stability analysis for infinitely small sand waves. Hereby, we investigate a steady current situation induced by a wind stress applied at the sea surface. The bed forms we find have wavelengths in the order of hundreds of metres when the resistance at the seabed is relatively large. The results show that it is possible to model the initial evolution of sand waves with a numerical simulation model. This paper forms the necessary first step to investigate the intermediate term behaviour of sand waves.     

The morphodynamic modelling of tidal sand waves on the shoreface

An artificial sand wave on the Dutch shoreface of the North Sea has been studied in conditions with relatively strong tidal currents in the range of 0.5 to 1 m/s and sediments in the medium sand size range of 0.2 to 0.5 mm. The sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km, respectively. The sand wave is dynamically active and shows migration rates of the order of a few metres per year. A numerical morphodynamic model (DELFT3D model) has been used to simulate the morphological behaviour of the sand wave in the North Sea. This model approach is based on the numerical solution of the three-dimensional shallow water equations in combination with a surface wave propagation model (wind waves) and the advection–diffusion equation for the sediment particles with online bed updating after each time step. The model results show that the sand wave grows in the case of dominant bed-load transport (weak tidal currents; relatively coarse sediment; small roughness height; low waves) and that the sand wave decays in the case of dominant suspended transport (strong currents, relatively fine sediment, large roughness height; storm waves).     

Formation of sand waves on unconsolidated sediments

Assuming that the long waves of sand found in sea beds are generated by stationary internal waves, a simplified analysis is made of their stability. The fluid model considered is two-dimensional and two-layered, with the lower layer stratified due to suspended sediments. Finally, the effect of flow over developed sand waves is studied under homogeneous conditions.     

Numerical modelling of liquefaction in loose sand deposits subjected to ocean waves

The failure of marine structures is often attributed to liquefaction in loose sand deposits that are subjected to ocean waves. In this study, a two-dimensional integrated numerical model is developed to characterize the liquefaction behaviours of loosely deposited seabed foundations under various types of ocean waves. In the present model, Reynolds-Averaged Navier–Stokes (RANS) equations are used to simulate the surface wave motion, and Biot's consolidation equations are used to link the solid-pore fluid interactions in a porous medium. A poro-elasto-plastic solution is used to reproduce foundation behaviour under cyclic shearing. Unlike previous investigations, both oscillatory and residual soil responses were considered; they are coupled in an instantaneous approach. Verification of the model results to the previous centrifugal wave tests is carried out, obtaining fairly good agreement. Numerical examples show that foundation behaviour under various types of wave loading, particularly standing waves or a solitary wave, embodies a completely two-dimensional process in terms of residual pore pressure development. The parametric studies demonstrate that liquefaction caused by the build-up of pore pressures is more likely to occur in loosely deposited sand foundations with poor drainage and under large waves.     

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS ¹⁴C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.     

The morphology and evolution of tidal sand bodies in the macrotidal Gulf of Khambhat,western India

Tidal sand bars and tidal sand ridges are extensively developed in the macrotidal Gulf of Khambhat, offshore western India. The inner and outer regions of the gulf are characterised by the development distinct tidal sand bodies with discrete geometries and dimensions. The outer gulf ridges are long, narrow, curvilinear and several metres high (∼20 m). They are asymmetric in cross-section and migratory in nature, forming ‘ribbon’ like sand bodies separated by tidal channels. Active dunes on these ridges indicate the presence of sand and their orientation parallel to palaeo-shorelines supports a tidal origin. In contrast to the outer gulf tidal sand ridges, sand bars associated with macrotidal estuaries flanking the Gulf of Khambhat typically have an elongate to diamond shape and are only hundreds of metres in width and a few kilometres length. These tidal sand bars occur in the estuary mouths and within the tidally influenced fluvial reaches of the rivers flowing into the gulf. The height of these sand bars is in the range ∼1–3 m. Due to high tidal ranges and bi-directional flow the sand bars do not develop significant height and are formed between the mutually evasive ebb and flow channels. Their bi-directional foresets and the presence of abundant mud drapes associated with the dunes within in-channel sand bars indicate a tidal origin.The Gulf of Khambhat acquired the present configuration in the last few thousand years since the Pleistocene sea-level lowstand (last glacial maximum, ∼18 ka) when the entire continental shelf was subaerially exposed and rivers down-cut into the coastal plain. With increasing sea-level rise, the exposed shelf was drowned, flooding parts of the Modern western Indian peninsula, and large tidal sand ridges formed in the outer gulf. After the fall of sea-level at 2 ka the gulf acquired the Modern configuration with multiple estuaries on both coastlines, rivers supplied the embayment with sandy sediment, and tidal sand bars formed in the Modern estuaries.Quantitative data gathered from the Modern Gulf of Khambhat indicates that for the P50 case, a vertical drill hole will encounter tidal sand bodies (ridges and bars combined) of approximate dimensions 1700 m long, 470 m wide and 1.5 m high, with a spacing of 400 m. In subsurface hydrocarbon reservoirs, where data is sparse and only limited amount of core is available, this quantitative dataset can be useful to constrain subsurface geocellular models. Also, the overall geometry, distribution and aspect ratio of the tidal sand ridges and tidal sand bars can be used to identify ancient counterparts through seismic geomorphology or in core.     

海底沙脊地貌的研究现状及进展

综述了国内外海底沙脊地貌的研究成果、技术方法以及东中国海沙脊研究中存在的问题。弓京港辐射沙脊群成因的研究依然是国内沙脊研究的焦点;东海中北部陆架沙脊地貌形成时期(海进与海退期)、沉积类型(现代、残留及潮流沉积)、沉积动力及沉积模式等研究尚存在较多争议;古潮流场的研究尚未得到足够重视。最新多波束探测数据显示,东海中南部外陆架60 m以深海域广泛分布线状沙脊群,其规模较前人预测的更大、范围更广,与长江口外喇叭状地形区中的沙脊地貌在空间分布特征和发展趋势上均存在差异。未来若能在利用多波束探测数据的基础上,对单道地震剖面、柱状样品以及钻孔等获取的多种资料进行综合分析,将更有利于对沙脊地貌的精细结构、时空展布规律以及成因机制和演化模式的研究。     

两相流模型在南海内孤立波与沙波相互作用中的应用

为研究内孤立波与沙波的相互作用,本文对基于OpenFOAM的SedWaveFoam求解器进行改进,建立了内孤立波-泥沙运动欧拉两相流模型。在利用试验资料对模型进行验证的基础上,在南海北部典型代表性条件下,模拟分析了500 m水深位置沙波床面上内孤立波作用下的水动力变化和泥沙运动。结果表明,内孤立波逐渐离开沙波时,海底沙波背流面处出现与内孤立波背景流速反向的流速,在内孤立波导致的流场作用下,沙波床面上的泥沙悬起并运动到床面以上的水体中。振幅100 m的内孤立波可以导致床面以上14 m高的位置处出现约0.07 kg/m³的悬沙浓度。     

曹妃甸二期围海造地工程取沙物理模型设计及验证

为解决曹妃甸首钢围海造地工程用沙问题,计划在曹妃甸甸头东侧滩涂取沙,因关系到曹妃甸岸滩的稳定性,拟通过波浪泥沙物理模型进行试验研究,探讨取沙对岸滩向/离岸运动的影响。基于河口海岸物理模型相似理论,探讨了模型比尺的确定方法及模型沙的选取;在缺乏合适验证资料的情况下,通过三种途径对物理模型的合理性和可靠性进行验证试验,验证试验表明,本模型可将二维沙滩剖面特性研究成果应用于三维条件。     

Tidal sand ridges and shoal-retreat massifs

In 1963, Off defined a bedform type which he described as rhythmic linear sand bodies caused by tidal currents. He figured twelve examples from around the world. Since then, the morphology and dynamics of sand transport in one of these areas, the tidal shelf seas around Great Britain, have undergone intensive study. The tidal sand ridges emerge as anomalies, in that they do not fit into the sequence of morphologic provinces which characterize the major sediment transport paths.It is suggested here that the ridge fields are analogous to the shoal-retreat massifs of the Middle Atlantic Bight in that they have been inherited from a nearshore regime during the course of the Holocene transgression. Shoal-retreat massifs are low, broad, shelf-transverse sand bodies which mark the retreat paths of coastal depocenters associated with littoral drift convergences. Two main types of shoal-retreat massifs in the Middle Atlantic Bight are: (1) estuarine shoal-retreat massifs; and (2) cape shoal-retreat massifs.Two similar classes of shoal-retreat massifs may develop in tidal shelf seas, but the mechanism is somewhat different. Class-1 tidal massifs are tidal ridge fields whose ridges were hydraulically packaged in an estuarine environment. If, upon transgression, they find themselves in a broad tidal bight which continues to funnel tidal flow, the ridges may survive for long distances out into the bight. The ridge fields of the Southern Bight of the North Sea may have undergone such an evolution.Class-2 tidal massifs occur off promontories in tidal seas that are swept by the edge waves generated by amphidromic tidal systems. Here the debris of shoreface erosion tends to be stored as shoreface-connected, tide-maintained ridges. Such ridges are also pre-adapted to survive with modification for long distances out on the associated shelf, as the water column deepens during a marine transgression.